1. Field of the Invention
The invention relates to a four-wheel-drive vehicle.
2. Description of Related Art
There has been conventionally known a four-wheel-drive vehicle in which driving force is always transmitted from a drive source (engine) to a pair of right and left main drive wheels (front wheels) and driving force is transmitted from the drive source to a pair of right and left auxiliary drive wheels (rear wheels) via a plurality of clutches (refer to Japanese Patent Application Publication No. 2012-61923 (JP 2012-61923 A)).
In the four-wheel-drive vehicle described in JP 2012-61923 A, a dog clutch is arranged between the drive source and a propeller shaft, which transmits driving force in the longitudinal direction of the vehicle. In addition, in the four-wheel-drive vehicle, a multi-disc clutch is arranged between the auxiliary drive wheels and a differential unit on the auxiliary drive wheel side, to which the driving force is transmitted through the propeller shaft. When the vehicle travels in a two-wheel-drive mode where the driving force is transmitted only to the main drive wheels, transmission of the driving force by the dog clutch and the multi-disc clutch is interrupted to suppress the rotation of the propeller shaft and the rotation of a differential case of the differential unit on the auxiliary drive wheel side. Thus, travel resistance (friction loss) is reduced and thereby fuel efficiency is improved.
In the four-wheel-drive vehicle described in JP 2012-61923 A, if the main drive wheels slip at the time when the vehicle starts in the two-wheel drive mode, it may not be possible to shift the vehicle into the four-wheel-drive mode because recessed portions and projected portions in the dog clutch cannot be engaged with each other. Therefore, there is provided a control device that controls the dog clutch such that the recessed portions and the projected portions are engaged with each other in advance before the vehicle starts when it is determined that the coefficient of friction of a road surface is low.
Such a four-wheel-drive vehicle may be started immediately after the drive source is actuated in response to a driver's operation of an ignition switch. In such a case, because the vehicle starts before the control device determines whether the coefficient of friction of the road surface is low, the dog clutch cannot be engaged before the vehicle starts. Therefore, when the main drive wheels slip, it may not be possible to converge the slip of the main drive wheels by shifting the vehicle into the four-wheel drive mode. The four-wheel-drive vehicle described in JP 2012-61923 A still has room for improvement in this respect.